Go to your control panel (MY CONTROLS link) and click on the MANAGE ATTACHMENTS link. Delete off some of your previously uploaded images. This will free up space for you.
This is the only way to gain more free space. Members get a total of 700KB for uploads, which is quite a bit considering how many members we have. The idea here to to rotate off your old content, making space for your new uploads.
Just an FYI: Supporting Members don't have a global upload limit. Not trying to be a pushy salesman here, just stating some options.